1. Field of invention
The invention relates to motion sensors, and in particular to a sensor that can detect dropping or tilting of an electronic device.
2. Related art
Present portable computers (e.g., laptop, palmtop, other electronic equipment that includes a microprocessor/microcontroller) and other electronic equipment are subject to extreme mechanical shock during operation. For example, the user may inadvertently drop a laptop computer during operation.
Disk drive units are especially vulnerable to mechanical shock. Proper drive operation depends on the drive machinery maintaining a very small gap between the read/write head and the recording material on the disk surface. When shock causes the read/write head to contact the disk, the head may be damaged and recording material removed from the disk. The contact may irretrievably destroy the head and/or the data, and render the drive inoperable. Thus it is important to predict mechanical shock so that the drive mechanism can position the head to avoid data surface contact (e.g., xe2x80x9cunloadxe2x80x9d the head by moving it outside the disk""s outer circumference (on a dynamically loaded drive), or position the head to the inner diameter within a data free landing zone (on a start/stop drive).
One source of shock is the sharp landing deceleration after a computer is dropped. The computer and its components briefly experience near zero acceleration (zero-G) during the immediately preceding free fall. A near zero-G sensor thus predicts a mechanical shock after a drop. In most cases a free falling computer will first strike a landing surface at a corner. A corner-first impact somewhat mitigates the shock to the computer because of the mechanical compliance of the typically plastic computer housing. The most severe shock occurs when the computer lands flat.
The computer may land flat after a free fall drop, or more likely after the user lifts one side of the computer and allows the side to slip from the hand. The computer pivots around the housing portion resting on the landing surface and the computer base strikes flat on the landing surface. This tilt and release type of drop is more difficult to detect than a free fall drop, hence the terminating shock pulse is more difficult to predict.
One method of anticipating the shock occurring after a tilt and release drop is to sense the computer""s tilt angle. A critical tilt angle is the angle above which the landing shock from the tilt and release drop exceeds the operational shock tolerance of a computer component (e.g., disk drive). When a tilt sensor detects such a predetermined critical angle, the computer may act to protect the disk drive from the potential shock if the tilt precedes a drop. When the tilt angle sensor detects that the computer housing tilt angle is below the predetermined angle, the computer returns the disk drive to normal operation. Detecting the computer tilt angle does not predict an imminent shock, but guards against the possibility. component protection based on tilt angle will have minimal impact on operation. For example, operators have difficulty typing at large keyboard angles (e.g., 30 degrees or more) and so drive access caused by the user""s keyboard inputs will be rare at higher tilt angles. Cautionary protective actions taken as the computer tilts above and below the critical will therefore not appreciably affect drive access performance during most computer operations.
Gyroscopic sensors detect only rotational acceleration, not static tilt. Rotation (or rotational acceleration) may or may not occur during a drop, and so a single gyroscope cannot act as a reliable drop predictor. Static tilt could be inferred by integration of rotational acceleration, but this approach would be very prone to errors resulting from, for example, low output signal levels for small movements, zero drift, and noise. Since gyroscopes have a single axis of sensitivity, three mutually perpendicular gyroscopes, along with their associated circuitry, would be required to cover the three orthogonal axes. This approach is expensive and consumes excess volume in small-scale equipment.
What is required is a small, inexpensive sensor that combines both near zero-G and tilt angle detection.
A bore is defined along an axis in a body. A mass subject to a magnetic attraction is placed within the bore and can travel a limited distance. During normal operation, the axis is approximately vertical so that the mass rests at one end (bottom) of the bore. A magnet is placed adjacent the opposite end (top) of the bore and imparts an upward attractive force on the mass. The magnitude of this upward force is established at a value that is less than the weight of the mass. An electrically conductive pickup coil (inductor) is placed near the end of the bore at which the mass normally rests. In some embodiments the coil is a conductive wire coil wound centered on the axis.
When the sensor experiences free-fall (drop), the gravitational attraction acting on both the sensor body and the mass within the bore are equal. The only attraction then acting on the mass is the magnetic attraction, and consequently the mass accelerates toward the magnet end of the bore and away from the inductive pickup coil. The mass movement causes an inductance change in the coil that is sensed by a detector circuit. The detector circuit outputs a signal that indicates the mass has moved, accordingly signifying that the body (and the device in which the sensor body is mounted) has been dropped. The output signal is used to trigger head movement to a position at which no damage will occur upon landing shock.
In a similar way, the mass moves upwards in the bore when the axis of the bore is tilted from the vertical beyond a certain critical angle. At that critical angle, the force from the magnet along the axis exceeds the component opposing force of gravity on the mass, and therefore the mass moves towards the magnet and away from the pickup coil. The detector circuit outputs a signal signifying that tilt beyond the critical angle has occurred.